The present invention relates to an improved voltage control circuit for a vehicle battery charging circuit and, more particularly, to a detecting circuit of a generator rise-up voltage in a voltage control circuit to prevent battery discharge even when an engine is stopped with the key switch being left closed.
Such a voltage control circuit is disclosed in a co-pending and co-assigned application Ser. No. 9359 filed on February, 1979.
The detecting circuit comprises a switching transistor for controlling field exciting current in accordance with rise-up voltage of a three-phase-Y-connected altenator windings having a full-wave rectifier. The base of the transistor is connected to the junction point of the three phase windings, in other words, the neutral point. When the alternator output voltage rises up, the voltage of the neutral point causes the switching transistor to be conductive to supply a full field exciting current.
However, in case the full-wave rectifier is subjected to salt water or muddy water, an electric leakage current path may be formed on the surface of the rectifier, through which a leakage current may flow from battery to the three phase windings, neutral point and to the input terminal of the transistor of the detecting circuit, with the result that the transistor may not control the field exciting current, when an engine is stopped with the key switch being left closed.